WEDNESDAY, July 27, 2022 (HealthDay News) — Researchers have long suspected that humans evolved to tolerate dairy products in order to reap their health benefits. Now, a new study disproves that idea.
About a third of the world’s population has an intestinal enzyme that allows them to digest lactose, a milk sugar. These lucky people – mostly of European descent – can feast on dairy products without suffering digestive problems.
However, the exact reason for the existence of these people has never been clear.
Almost all babies can easily digest milk thanks to an enzyme called lactase. But for prehistoric humans, this enzyme would naturally decline and stop in adulthood – what scientists call lactase non-persistence.
That was, at least, until several thousand years ago when a genetic variant emerged that allowed some adults to continue producing lactase.
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Today, nearly two-thirds of Americans have this ability, according to the US National Institutes of Health. However, most of the world’s population still lacks the enzyme lactase, including most adults in Africa and Asia.
Experts have long theorized that lactase persistence evolved so adults could drink animal milk, presumably for nutritional and health benefits. Such a development could have been particularly useful for northern Europeans, who would have been at greater risk of having fragile bones.
But in the new study, published July 27 in the journal Nature, scientists have found that prehistoric Europeans widely consumed milk thousands of years before genetic lactase persistence emerged.
They are based on an analysis of 7,000 remains of archaeological pots, which allowed them to detect the milk fats absorbed in the ancient pottery.
Researchers estimate that European farmers commonly consumed milk 9,000 years ago (around 7,000 BC) – when it’s unlikely any of them were genetically endowed to produce lactase.
Scientists base the latter hypothesis on published genetic data from nearly 1,800 prehistoric European and Asian individuals.
They found that the gene variant for lactase persistence was not detectable until about 4600 BC, and did not become common until about 2000 BC.
All of this suggests that prehistoric humans drank milk long before any genetic changes around lactase production. So it seems that despite the enzyme deficiency, they were able to handle the milk without too much trouble, according to researcher Mark Thomas, professor of evolutionary genetics at University College London.
For these ancient populations, he said, milk from farm animals would have provided convenient, nutrient-dense food — likely worth gas, bloating, or even mild diarrhea.
And, Thomas said, it’s not even known how often or to what extent prehistoric people would have suffered from these symptoms.
Other factors, he said, could influence whether someone lacking the enzyme lactase actually exhibits symptoms related to drinking milk, including the makeup of resident bacteria in the gut.
In fact, Thomas said, a separate part of the study illustrates this well.
To do this, the researchers combed through data from the UK Biobank, a research project collecting genetic and medical information on around 500,000 British adults.
Of all the Biobank participants who genetically lacked the enzyme lactase, few followed a lactose-free diet and 92% described themselves as milk drinkers. This was almost identical to the rate in genetically lactase persistent people.
So, Thomas said, it’s likely that — as in prehistoric times — many people with lactase deficiency today don’t suffer from chronic symptoms.
A dietician who was not involved in the study said that there is indeed a wide variation in symptoms of lactose intolerance.
Many people can handle modest amounts of sugar, like a glass of milk a day, said Emma Laing, director of dietetics at the University of Georgia and spokesperson for the Academy of Nutrition and Dietetics.
Additionally, she says, people who are lactose intolerant often do well with dairy products like yogurt and hard cheese, which are relatively low in lactose.
Laing also agreed that having a good amount and diversity of “healthy” gut bacteria can help digest lactose, so people may not experience symptoms from this ice cream dish.
But if humans haven’t developed a lactose tolerance to enjoy dairy, why have we?
Using statistical modeling, Thomas and his colleagues found support for two main drivers of lactose tolerance.
One is starvation: researchers believe that during times of poor harvests and food shortages, people relied heavily on milk. If it caused diarrhea in a malnourished person, it could prove fatal. So those few people with genetic lactase persistence might have been more likely to survive and pass on that gene.
The other possible factor is the increasing exposure of humans to infections as population density increases. Again, according to the researchers, any diarrhea caused by drinking milk – although unpleasant for a healthy person – could be fatal for someone with an infectious disease. And again, this would promote the survival of people who are genetically able to produce lactase.
But while many prehistoric people may have enjoyed their cup of milk, Laing warned that lactose intolerance can cause significant symptoms in some people. And unlike 7000 BC, she noted, lactose is now hiding in many processed and prepared foods, and even in medicines.
So individuals vary, Laing said, in what steps they need to take to avoid symptoms — which they should manage with the help of their doctor or dietitian.
The US National Institute of Diabetes and Digestive and Kidney Diseases has more on lactose intolerance.
SOURCES: Mark Thomas, PhD, professor, evolutionary genetics, University College London, UK; Emma Laing, PhD, RDN, director, dietetics, University of Georgia, Athens, and spokesperson, Academy of Nutrition and Dietetics, Chicago; Nature, July 27, 2022, online